In the building industry, it is often necessary to raise a building structure, e.g. to raise a riverside or seafront, building above flood or high-tide level. A typical example of this is the city of Venice, where the ground floors of buildings are regularly flooded by so-called “high-water phenomena”.
Alternatively, a building may be raised to build a basement underneath, in situations in which excavating underneath the building is undesirable or impossible, or to increase the height, to make full use, of a floor.
Patent IT1303956B proposes a method of raising a building structure, whereby a new foundation for the building structure is constructed with a number of through holes and, for each through hole, a connecting member projecting at least partly upwards and fixed to the foundation, next to the hole; next, a pile is inserted through each hole, and a first thrust is exerted statically on the pile to drive it into the ground (the first thrust is applied by a thrust device located over and cooperating with a top end of the pile, and connected to the projecting part of the connecting member, which acts as a reaction member for the thrust device when driving in the pile). Once all the piles are driven into the ground, a second thrust is applied statically between each pile and the foundation to raise the building structure with respect to ground; and, after the lift, each pile is fixed axially to the foundation.
Patent Application WO2006016277A1 proposes a method of raising a building structure resting on a supporting body in turn resting on the ground, whereby a new foundation of the building structure is constructed with a number of through holes and a number of connecting members, each fixed to the foundation close to a hole; next, a pile is inserted through each hole, so that the bottom end of the pile rests on the supporting body, and the top end projects from the hole. At this point, each pile is attached to a thrust device resting on the top end of the pile on one side, and connected to the corresponding connecting member on the other side; and thrust is applied statically to the piles by means of the thrust devices to raise the building structure with respect to the supporting body. Once the lift is completed, each pile is fixed axially to the foundation. The main difference between the methods described in Patent IT1303956B and Patent Application WO2006016277A1 lies in Patent IT1303956B driving each pile into the ground individually before commencing the lift, whereas, in Patent Application WO2006016277A1, given the existing supporting body between the building structure and the ground, lifting is performed without driving the piles into the ground first.
In the case of building structures of considerable size and/or particular structural situations, the above known lifting methods leave room for improvement. That is, during the lift, the structure may be subjected to severe stress requiring major consolidation work.
To reduce stress on the building structure during the lift, Patent Application WO2007138427A2 proposes dividing the lifting devices into three equal, symmetrical, independent work groups. During the lift, the lifting devices of only one work group at a time are operated simultaneously, while those of the other two groups are left idle, so the building structure is raised isostatically.
For the lift method in Patent Application WO2007138427A2 to work properly, the three work groups must be as equal as possible, i.e. comprise roughly the same number of lifting devices and be as symmetrical as possible, i.e. the thrust barycentres of the three work groups must correspond as closely as possible to the vertices of a preferably equilateral triangle with its centre at the barycentre of the weight of the building structure and the mat to be lifted. For the lift method in Patent Application WO2007138427A2 to work properly, the barycentre of the weight of the building structure to be lifted must therefore be determined accurately.
In certain situations, however, it is not easy to accurately determine the barycentre of the weight of the building structure (especially in the case of historic buildings, in which the actual consistency and hence weight of the walls is difficult to assess) or to divide the lifting devices into three equal, symmetrical groups (especially in the case of buildings with an irregular plan). In some situations, therefore, the lift method in Patent Application WO2007138427A2 proves difficult to implement.
In the patent applications referred to above, each through hole in the new mat is lined with a metal guide tube having an annular anchoring flange to which the stays are fixed. When driving in the foundation pile, the shaft of the foundation pile slides axially with respect to the guide tube integral with the mat, and likewise when raising the building structure. It has been observed, however, that whereas, when driving in the foundation pile, the shaft of the foundation pile slides freely with no problems along the guide tube, even severe sliding friction may occur between the shaft of the foundation pile and the guide tube when raising the building structure.
This sliding friction is particularly harmful by producing random, unpredictable, localized irregularities in the lifting process, in turn resulting in even severe stress within the mat. It may even result in damage (i.e. uncontrollable deformation) of the foundation pile shaft, thus impairing performance of the pile. The main reason for this sliding friction between the foundation pile and the guide tube is that, when driving in the pile, plastic concrete is pressure-injected beneath the mat to fill the tubular channel formed by the wider foot of the pile as it sinks into the ground, and may leak into the gap between the foundation pile shaft and the guide tube. When driving in the pile, the concrete is still fresh, i.e. highly liquid, and so produces no significant friction as the shaft slides along the guide tube. On the other hand, during the lift (which normally takes place at least a month after the piles are driven), the concrete between the shaft and the guide tube has set and may produce even severe friction as the shaft slides along the guide tube.